Most conventional elastomers must be crosslinked, i.e., cured or vulcanized, in order to obtain suitably strong, shaped articles. However, crosslinking introduces relatively permanent bonds among the rubber's polymer molecules. Once these bonds are formed, they prevent adequate flowability for subsequent processing or molding. Thus, it is often desirable to process and mold elastomeric articles prior to curing.
Prior to being crosslinked, however, these elastomers often lack sufficient strength, also known as green strength, for easy handling and processing. Green strength refers to the cohesiveness and dimensional stability under relatively low stress of a rubber compound before it is vulcanized or cured. Green strength is important in such industries as tire manufacturing, where the innerlining and other layers of the tire may be formed in one stage, while curing takes place in a later stage. Green strength is important in allowing the manufacturer to shape and stretch the uncured elastomer, while maintaining other favorable properties such as low air permeability.
Ionomerization (i.e., ionomer formation) provides a means for modifying green strength without chemical crosslinking (i.e., formation of covalent bonds) through a cure process. Ionomerization of amines is also known in the art as “quaternization”, due to the formation of a quaternary amine (ammonium ion). Since it is a reversible process, ionomerization provides green strength at lower temperature as well as good processability at higher temperatures. A typical method of ionomerization is the addition of tertiary amines to the elastomer compositions, the basic amine functionality interacting with a more acidic functional group on the polymer in order to form stable interactions that increase green strength. U.S. Pat. Nos. 3,898,253 and 4,256,8576 generally describe the use of tertiary amines to directly modify halobutyl rubber compositions to improve their green strength. However, halobutyl rubber, such as bromobutyl rubber, is not very reactive with these amines. Therefore, the mixture must be heated for a substantial period of time in order to obtain sufficient green strength.